Building block



FIG. 1 is a top perspective view of a building block showing my design;

FIG. 2 is a front elevational view thereof with the rear, right and left elevational views being mirror images thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom plan view thereof; and,

FIG. 5 is a bottom perspective view thereof. 

CLAIM The ornamental design for a building block, as shown and described. 